The invention relates to a tunable filter arrangement. The invention further relates to a transmitter, a receiver, a mobile telephone device, and a cordless data transmission system with a tunable filter arrangement, as well as to a tunable bulk acoustic wave resonator.
The stormy developments in the field of mobile telephony and the continuous miniaturization of cordless telephone devices lead to higher requirements being imposed on the individual components. Thus a high selectivity in the high frequency part is necessary for protecting the receiver from the rising number of potentially interfering signals from other systems. This is achieved, for example, by means of bandpass filters which transmit only a limited frequency band and which suppress all frequencies above and below this band.
At the present moment, filters with ceramic electromagnetic resonators are among the means used for this purpose. A miniaturization of these filters, however, is limited by the electromagnetic wavelength. So-called surface acoustic wave (SAW) filters built up from surface acoustic wave resonators can be given a considerably smaller construction. This is because the acoustic wavelength is smaller than the electromagnetic wavelength by 4 to 5 orders of magnitude. A surface acoustic wave resonator comprises a piezoelectric layer on which finger-shaped electrodes are provided. A signal applied to the input electrodes excites the piezoelectric material into mechanical vibrations, which propagate in the form of acoustic waves on the upper side of the layer and are converted back into an electric signal again by the output electrodes.
An alternative is formed by bulk acoustic wave (BAW) filters comprising bulk acoustic wave resonators. Bulk acoustic wave filters have advantages as regards their size, power, and IC compatibility. Bulk acoustic wave resonators are built up from three components in principle. The first component generates the acoustic wave and comprises a piezoelectric layer. Two electrodes arranged above and below the piezoelectric layer represent the second component. The third component has the task of acoustically insulating the substrate from the vibrations generated by the piezoelectric layer.
It is an interesting aspect that the properties of a resonator or filter can be varied. This may be done, for example, through coupling of a resonator or filter with a varicap diode. It is a disadvantage of the combination of active and passive components that the active components may be contaminated by the materials of the passive components during the manufacture of the resonator or filter.
An alternative possibility is disclosed in U.S. Pat. No. 5,446,306. This describes a semiconductor bulk acoustic wave resonator and a semiconductor bulk acoustic wave filter which comprises a semiconducting substrate, a first and second electrode, and arranged therebetween a piezoelectric layer of AlN or ZnO. The resonance frequency of the resonator is changed in that a DC voltage is applied to the electrodes.
The invention has for its object to provide a tunable filter arrangement which can be manufactured in a simple and inexpensive manner.
This object is achieved by means of a tunable filter arrangement which comprises a substrate and provided thereon an arrangement of at least two mutually coupled resonators of which at least one is connected to a capacitor with tunable capacitance.
The electrical properties of the resonator, for example its resonance frequency or its anti-resonance frequency, can be changed through coupling of a resonator to a capacitor. If the resonator is present in a filter arrangement, these changes will influence the overall filter characteristic. Since the capacitor has a tunable, i.e. changeable capacitance value, these changes can be effected more or less strongly.
It is preferred that the capacitor comprises a dielectric of a material having a voltage-dependent relative dielectric constant ∈r.
Certain materials have a dielectric constant ∈ which is strongly dependent on an applied voltage. When a DC voltage is applied to the first and the second electrode of a capacitor having a dielectric with a voltage-dependent relative dielectric constant ∈r, the value of the dielectric constant ∈r will drop, and thus the capacitance of the capacitor. This also changes the influence of the capacitor on the electrical properties of the resonator to which it is coupled.
It is particularly highly preferred that the material with a voltage-dependent relative dielectric constant ∈r is chosen from the group comprising PbTi1xe2x88x92xZrxO3 (0xe2x89xa6xxe2x89xa61) with and without dopants of La, Nb or Mn with and without excess lead, BaTiO3 with and without dopants, SrTiO3 with and without dopants, Ba1xe2x88x92xSrxTiO3 (0xe2x89xa6xxe2x89xa61) with and without dopants of Ca and Pb, Ba1xe2x88x92xSrxTiO3 (0xe2x89xa6xxe2x89xa61)+MgO, Ba1xe2x88x92xSrxTiO3xe2x80x94Pb1xe2x88x92yCayTiO3 (0xe2x89xa6xxe2x89xa61, 0xe2x89xa6yxe2x89xa61), CaTiO3 doped with Bi, Srn+1TinO3n+1 (1xe2x89xa6nxe2x89xa65), Pb1xe2x88x92xCaxTiO3 (0xe2x89xa6xxe2x89xa61), Ba1xe2x88x92xSrxTiO3 (0xe2x89xa6xxe2x89xa61) with and without added VOx (1xe2x89xa6xxe2x89xa62.5) and/or SiO2, Ba1xe2x88x92xSrxZryTi1xe2x88x92yO3 (0xe2x89xa6xxe2x89xa61, 0xe2x89xa6yxe2x89xa61) with and without dopants, Ba1xe2x88x92xPbxTiO3 (0xe2x89xa6xxe2x89xa61) with and without excess lead, Ba1xe2x88x92xCaxTiO3 (0xe2x89xa6xxe2x89xa61), SrZrxTi1xe2x88x92xO3 (0xe2x89xa6xxe2x89xa61) with and without dopants, [PbMg1/3Nb2/3O3]xxe2x80x94[PbTiO3]1xe2x88x92x (0xe2x89xa6xxe2x89xa61), (Pb,Ba,Sr)(Mg1/3Nb2/3)xTiy(Zn1/3Nb2/3)1xe2x88x92xxe2x88x92yO3 (0xe2x89xa6xxe2x89xa61, 0xe2x89xa6yxe2x89xa61), Pb1xe2x88x92xCaxTiO3 (0xe2x89xa6xxe2x89xa61), (Ba1xe2x88x92x+y/8Srx+y/8)2Na1xe2x88x92yNb5O15 (0xe2x89xa6xxe2x89xa61, 0xe2x89xa6yxe2x89xa61) with and without excess Na+, (Ba1xe2x88x92x+y/8Srx+y/8)2K1xe2x88x92yNb5O15 (0xe2x89xa6xxe2x89xa61, 0xe2x89xa6yxe2x89xa61) with and without excess K+, (Ba1xe2x88x92xSrx)2K1xe2x88x923ySEyNb5O15 (0xe2x89xa6xxe2x89xa61, 0xe2x89xa6yxe2x89xa61, SE=ion from the group of rare earths), Sr2Ba4Ti2Nb8O30, BiNbO4 with and without VOx (1xe2x89xa6xxe2x89xa62.5) and/or CuO dopants, (Bi2xe2x88x92xZnx)(Nb2xe2x88x92yZny)Ox,
Bi2(Zn1/3Nb2/3)2O7,
a) Pb(Mg1/2W1/2)O3,
b) Pb(Fe1/2Nb1/2)O3,
c) Pb(Fe2/3W1/3)O3,
d) Pb(Ni1/3Nb2/3)O3,
e) Pb(Zn1/3Nb2/3)O3,
f) Pb(Sc1/2Ta1/2)O3,
combinations of the compounds a) to f) with PbTiO3 and/or Pb(Mg1/3Nb2/3)O3 with and without excess lead and Ba1xe2x88x92xZrxTiO3 (0xe2x89xa6xxe2x89xa61).
These materials show a particularly strong dependence of their dielectric constants ∈ on an applied voltage.
It is preferred that the resonators are chosen from the group comprising bulk acoustic wave resonators, surface acoustic wave resonators, and LC resonators.
Filter arrangements comprising bulk acoustic wave resonators or surface acoustic wave resonators can be manufactured with a high quality factor Q and a high coupling factor k. LC resonators can be manufactured in a simple manner.
It is particularly preferred that the resonators are constructed in a thin-film technology process.
A construction of the resonators in thin-film technology on a substrate renders it possible to obtain such a filter arrangement with small dimensions.
It is particularly highly preferred that a bulk acoustic wave resonator comprises a resonator unit of a lower and an upper electrode as well as a piezoelectric layer arranged therebetween and a reflection element arranged between the substrate and the resonator unit.
Such a bulk acoustic wave resonator can be manufactured without cumbersome lithographic processes because the resonance frequency of the resonator is defined by the layer thickness of the piezoelectric layer. In addition, such a bulk acoustic wave resonator is clearly more robust that other types of bulk acoustic wave resonators such as single-crystal resonators, resonators with membranes, or resonators with an air gap.
In a preferred embodiment, the resonator connected to a capacitor of tunable capacitance is also connected to a further capacitor.
It is particularly highly preferred in this embodiment that at least one capacitor is connected in series with and at least one capacitor is connected in parallel to the resonator.
This embodiment offers a particularly wide range of possibilities for varying the filter characteristic.
The invention furthermore relates to a transmitter, a receiver, a mobile telephone device, and a wireless data transmission system provided with a tunable filter arrangement which comprises a substrate and provided thereon an arrangement of at least two mutually coupled resonators of which at least one is connected to a capacitor with tunable capacitance.
The invention furthermore relates to a tunable bulk acoustic wave resonator which comprises a substrate and provided thereon a resonator unit with a lower and an upper electrode as well as a piezoelectric layer arranged therebetween, and a reflection element which is arranged between the substrate and the resonator unit, which resonator is connected to a capacitor having a tunable capacitance value.
The electrical properties of the bulk acoustic wave resonator can be trimmed over a wide range through coupling of the resonator to a tunable capacitor.